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PostPosted: Sun Jul 17, 2011 10:12 am 
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Hi folks,

Smaller pixels demand smaller f-numbers if the lens is to keep up and diffraction softening doesn't become an issue:
      F-number.....Resolution [μm]
      .....1.0................0.67
      .....1.4................0.94
      .....2.0................1.34
      .....2.8................1.88
      .....4.0................2.38
      .....5.6................3.76
      .....8.0................5.37
      .....11.................7.38
      .....16................10.74
      .....22................14.76
      .....32................21.47
The table is for green light and I found it in a rather difficult to read translation of a commentary about a Sony patent describing how to beat the diffraction limit which you can read here.

This isn't a variation on the negative refractive index "superlens" (Wiki) but relies instead on the use of a high refractive index material in essentially direct contact with the sensor. As such it would seem to be just a variant of Immersion Lithography (Wiki), a technique now being used to make the latest computer chips.

The gain in resolution (or abatement of diffraction if you prefer) is proportional to the refractive index of the liquid which for water is only about 1.33 at the wavelengths we are interested in (source), useful but it'd be a real drag having to top up one's camera with water every time one changed lenses. :twisted:

It's a rainy morning and I try to avoid liquid filled lenses so I thought I'd stay in the dry and try to find out just how high a refractive index can go? For diamond the answer is about 2.42 and while it might avoid the need to carry a water bottle around it wouldn't be practical, even at Leica prices! The record refractive index I've come across is 38.6 :shock: (source). Unfortunately the metamaterial used only exhibited this at a frequency of about 0.3 THz which is about 1/1000th of the frequency of visible light so it's of no use to us.

Might we see liquid filled compact cameras in the future in an attempt to improve optical performance? I don't know and I would guess that one potential deal breaker would be a possible need to avoid significant air filled spaces anywhere in the lens train. That would mean that zooming would either not be possible (commercially a non starter for compacts) or use of a liquid lens which can be deformed as required. Chromatic aberration might be correctable via "in camera" processing, I suppose, but would it be practical?

The commentary I linked to initially does acknowledge these issues and, rather tongue in cheek perhaps, mentions that this technology might be suitable for a Fujifilm X100 class of camera. Maybe so or maybe not. The big question is whether anyone can bend this technology into a useful product. Any ideas?

Bob.

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PostPosted: Sun Jul 17, 2011 10:39 am 
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Sony apparently are working on liquid lenses but that's more for changing their shape.

I think creating a sealed liquid filled lens would be possible. Look at the fixed length lenses we already have that don't change length on zoom or focus.

The gap between lens rear and sensor would be more difficult, where I wonder if a small air gap away from the sensor would be acceptable to allow a condensing module to be fitted over the sensor? That module would be fixed in contact with the sensor, with an input opening designed such that it would not itself be a limiting factor. I don't know how allowable air is at any point between the final lens element and sensor, but worst case would the subject need to be underwater too? :D

I think I'm way over my head here, and not just with water...

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PostPosted: Sun Jul 17, 2011 1:43 pm 
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Hi popo,

Me too. The more I think about light the more bizarre it seems and we spend most of our days immersed in the stuff. I explored one aspect of this strangeness in my thread What is the "width" of a photon?. And now I learn that the limit on resolution isn't only dependent on the diameter of the inlet pupil but also on the local speed of light behind the initial optical surface.

In telescope terms suppose during the best seeing one could just separate the two components of a double star with a four inch refractor. Until today I had assumed that such a limit to resolution was set in stone and could only be improved by using a bigger telescope or, less realistically, a shorter wavelength of light. Not so as it now seems that if the lens could somehow be made of solid diamond and extended all the way back from the front surface to the imaging sensor then one could either resolve stars about 2.4 times closer together under perfect seeing or use a 1.7" telescope for the same resolution as a conventional four incher.

So where and how exactly does this loss of information occur when there's a lot of air between the lens and the sensor? In the case of my thought experiment solid diamond telescope, and echoing your own excellent question, just how far from the sensor does that rear diamond surface have to get before one loses the 2.4x enhanced resolution that diamond's high refractive index provides?

Where's our resident professor of optics when one needs him?

Bob.

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Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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PostPosted: Sun Jul 17, 2011 2:20 pm 
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He's flat out building the mini-WASP array in preparation for a "First Light" party next month. He's also just back from the Starmus Festival in Tenerife - an event I call "Woodstock" for astronomers with an unbelievable line up of megastars - one of which is shown below:

Image

Anyway - enough of that. There are tons of materials with an index close to diamond that are used extensively in optics - Tantalum Pentoxide is one with an index of 1.96 which I used to make photonic crystals. Problem with using high index materials for your lens of course is the huge Fresnel reflection you get at the air/material interface, so then you need to put a multilayer antireflection coating dielectric on.

There are many "tricks" that can be used to improve the capabilities of image sensors, but the problem is that chip manufacturers do not want to bring "non Silicon-standard" materials into their fab-lines, for very good reasons.

Greg

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